feat: trade analysis equity overlays, theme settings, chart fixes

Trade Analysis:
- Add toggleable equity curve overlays: Account total, By Strategy,
  By Symbol, By Day of Week — checkboxes above each chart
- Fix all chart backgrounds to transparent (inherit page theme)
- Fix grid and font colours to work on both dark and light backgrounds

Settings page:
- Full theme control: Dark / Light / Custom radio with Apply button
- Custom mode: colour pickers for accent, page bg, sidebar bg, text,
  font selector and base dark/light toggle
- Preview swatches before applying
- Discard / Reset to Dark button
- Text size: Normal / Large (+2px) / Extra Large (+4px) via CSS injection
- inject_theme_css() helper for app.py to apply font size on all pages
- Theme written to .streamlit/config.toml on apply

.streamlit/config.toml

@@ -0,0 +1,7 @@
+[theme]
+base = "light"
+primaryColor = "#2E75B6"
+backgroundColor = "#ffffff"
+secondaryBackgroundColor = "#f0f2f6"
+textColor = "#1a1a1a"
+font = "sans serif"